This invention relates to electrically controlled pattern tracers of the type which have means for selectively varying the lateral offset of a tracing element with respect to a pattern for different kerfs associated with different cutting tools, such as disclosed in patent application Ser. No. 428,577 of Grant et al. filed Sept. 30, 1982, and entitled "Pattern Tracer With Variable Effective Forward Offset and Method", assigned to the assignee of the present application and, more particularly, to such an apparatus with means improving the tracing of a pattern turn by controlling the movement of the tracing element differently depending upon the the kerf.
Sharp, or small radius, turns in a pattern can be more closely followed by adjusting the forward offset and tracing speed for a given kerf. However, the lateral offset of the scanning mechanism for a given kerf produces deviation from the pattern when cutting out a corner with a small radius. This deviation increases as the kerf or lateral offset of the tracing element is increased.
In cutting inside corners on a pattern, two different type of errors occur depending on the kind of scanning mechanism used. First, mechanical scanners that are laterally displaced from the center of the tracing mechanism can permit the tracer to move too far into the corner before detecting the corner. This excessive movement of the tracer into the corner causes a gouge to be taken out of the material being cut. An example of such a gouge is shown by the dotted line in FIG. 1, where the tracer element has traveled along the pattern line, from point A to a pattern turn at point C and has departed from the pattern en route to another point D. For purposes of simplification, the location of the tracing element is shown on the pattern line in FIG. 1 and other figures but it should be appreciated that, in actual practice, it follows the line along a path which is laterally offset from the line by an amount determined by pattern line thickness and kerf. Secondly, electronic, nonrotating scanners will detect the corner too early which will cause the corner to be rounded off, as illustrated by the dotted line in FIG. 2.
In cutting outside corners, regardless of whether a mechanical scanner, or electronic nonrotating, scanner is used, the corner is detected too early. As a result, the tracer turns the corner too early and causes the corner to be cut off or rounded, as illustrated by the dotted line in FIG. 3.
Another similar problem arises when the tracing element is "catching" a pattern by approaching it from an off pattern position and then first detecting, or "catching", the pattern, and thereafter, beginning to trace the pattern. Regardless of the type of scanning mechanism, the tracer will move too far into the pattern before tracing the pattern and thereby gouge the material to be cut before the proper kerf is implemented. This is shown in FIG. 4. The pattern tracer approaches the pattern line determined by points A, B, C and D, and then the tracer detects the pattern too late before the momentum of the tracer can be shifted to place the tracer onto the pattern and implement the kerf. As a result, a gouge will occur in the material as shown by the dotted line.
Because of these cornering problems, tracer patterns are usually designed to have the minimum radius of all pattern corners to be not less than twice the maximum kerf to be used on the pattern. As a result, patterns with zero radius or small radii turns cannot easily be accurately cut with present tracers, particularly if kerf is also provided.
Some of these problems have been alleviated by the pattern tracer disclosed in the aforementioned U.S. patent application of Grant et al., U.S. Ser. No. 478,577. In the pattern tracer discussed in this application, a large forward offset is used and the information received about the advancing pattern is stored in a computer memory. When a corner is detected, the stored information is used after a time delay which is a function of the velocity of the tracer. The amount of delay is stored in another part of the computer memory called a scan delay table. The scan delay numbers in this table are based on the forward offset and velocity of the tracer and are used to substantially maintain the tracer on the pattern when turning a corner. However, this table of numbers was not dependent upon the lateral offset required for kerf and minor tracing errors therefore still exist.